1. Field of the Invention
The present invention relates to compact imaging lenses used in digital still cameras including solid-state imaging elements, such as Charge Coupled Devices (CCD), and more specifically, to an imaging lens suitable for use in a camera installed in, for example, a mobile phone, a portable personal computer, a personal digital assistant like a portable music player, etc.
2. Description of the Related Art
In the mobile-phone market, models in which cameras are installed, that is, camera-equipped mobile phones, constitute the majority of the market. In particular, models including imaging lenses that have an angle of view (total angle of view) of 60° to 75° and that can be used together with CCDs having one to two million pixels have come into widespread use.
Recently, however, in order to add values to the products, imaging lenses that can be used together with CCDs having a larger number of pixels have been demanded. On the other hand, in solid-state imaging elements, such as CCDs, noise is generated if light is continuously incident thereon while an image is being recorded. Therefore, as the number of pixels in the CCDs is increased, it becomes necessary to block light with a shutter or the like.
Accordingly, recently, small, thin camera-equipped mobile phones including CCDs having large numbers of pixels and equipped with shutters have been preferred.
However, to install a shutter while satisfying the demand for thickness reduction, a space for accommodating the shutter must be provided while reducing the length of the optical system. In addition, aberrations must be corrected. This is difficult for known imaging lenses.
For example, an imaging lens is known which includes, in order from an object side, an aperture stop, a first lens with a negative refractive power having a meniscus shape with a convex surface on the object side, a second lens with a positive refractive power cemented to the first lens, a third lens with a positive refractive power having a meniscus shape with a concave surface on the object side, and a fourth lens with a weak negative refractive power having an aspherical surface on at least one side thereof (see, for example, Japanese Unexamined Patent Application Publication No. 2004-184987).
In this imaging lens, the aperture stop is positioned in front of the first lens. Therefore, when a shutter is installed, a large shutter must be used to block the incident light. Thus, it is difficult to reduce the overall size.
In addition, another imaging lens is known which includes, in order from an object side, a first lens with a positive refractive power having a meniscus shape with a convex surface on the object side, an aperture stop, a second lens with a positive refractive power having a meniscus shape with a concave surface on the object side, and a third lens with a negative refractive power having a concave surface on an image-plane side (see, for example, Japanese Unexamined Patent Application Publication No. 2003-322792).
In this imaging lens, the first lens has a concave surface on the image-plane side and the second lens has a concave surface on the object side. Therefore, the distance between rim portions (edges) of the lenses is small.
Accordingly, in this imaging lens, it is necessary to increase the distance between the first lens and the second lens along an optical axis thereof to place a shutter. Thus, it is difficult to reduce the total lens length.
In addition, still another imaging lens is known which includes a first lens with a positive refractive power having a biconvex shape, an aperture stop, a second lens with a positive refractive power having a convex surface on an object side, and a third lens with a negative refractive power having an aspherical surface on each side thereof, the surface on the object side of the third lens being convex in an area around an optical axis (see, for example, Japanese Unexamined Patent Application Publication No. 2004-302060).
In this imaging lens, although the first lens has a biconvex shape, negative refractive power is reduced because the third lens has a convex surface on the object-side. Therefore, it is difficult to reduce the total length of the lens system, i.e., the distance from the object-side surface of the first lens to an image plane.